Technical Field
This invention in general relates to gas monitors used, for example, in the process industry. In particular this invention relates to improvements in detection and measurement of gas concentrations and gas emissions based on tunable diode lasers.
Description of the Related Art
Accurate monitoring of gaseous species at low concentrations is required for a wide range of industrial, regulatory, and academic fields. The most common include atmospheric chemistry, pollution monitoring, industrial process monitoring and control, safety, breath analysis, and agricultural research. One of the most reliable principles for continuous monitoring of gases is the measurement of gas absorption since most gases have one or more absorption lines in the ultra violet, visible or the infrared part of the spectrum. This technique is known as absorption spectroscopy. With this method a beam of light such as a laser beam that is absorbed by the gas of interest, is directed through the gas or a mixture of gases. The degree of absorption of the light beam is then used as an indicator for the concentration of the gas to be detected. Many different spectroscopic techniques exist, but the use of single line spectroscopy utilizing single mode tunable diode lasers is probably the one giving best sensitivity and selectivity due to its high spectral resolution involving a low risk of interference from other gases.
There are two popular spectroscopic methods of laser gas detection. In one the frequency of the laser is rapidly scanned across the gas absorption line by modulation of the laser diode current. Gas absorption results in modulation of the amplitude of the transmitted light and this amplitude can be measured using a photodetector and simple electronics. The absorption of the laser beam on-line and off-line may be compared and the gas absorption and concentration computed. This is method is referred to by several names including scanned direct absorption and rapid scan absorption. This method has the advantage of simplicity but it can be difficult to establish a zero absorption baseline. The other popular method is called modulation spectroscopy; the most commonly used is referred to as wavelength modulation spectroscopy (WMS). In this method the laser frequency and amplitude is modulated using laser current as in the case of direct absorption. In addition, the laser current is also modulated at a second relatively high frequency. Gas absorption distorts the amplitude of the modulated laser light so that harmonics of the high modulation frequency appear after the beam has passed through a gas. These harmonics are measured by demodulating the gas signal. Sensitive tunable diode laser (TDL) absorption measurements have been performed for decades with wavelength modulation spectroscopy (WMS) for a wide variety of practical applications. With its better noise-rejection characteristics through laser wavelength modulation strategies, WMS has long been recognized as the method of choice for sensitive measurements of small values of absorption, and thus is favored for trace species detection.
Laser diode wavelength stability is vital in tunable diode laser spectroscopy (TDLS). Since both laser diode threshold current and laser emission wavelength are functions of temperature, laser diode temperature stability is very important in laser spectroscopy. For example, the commonly used 1651 nm atmospheric absorption line of methane has a linewidth (HWHM) of 50 picometers (pm). Laser spectroscopy requires the wavelength precision of the laser to be substantially less than this linewidth.
TDLS gas sensing systems, the laser diode temperature is controlled with a thermoelectric cooler (TEC). The laser die is typically mounted in close proximity to a Peltier element and a temperature-sensing thermistor. The TEC controlling circuit uses current from the thermistor in a feedback loop with the Peltier element to regulate temperature of the thermistor and the laser diode. It is possible and practicable to regulate the temperature of the Peltier element to less than 1 milli-kelvin. However, even with good thermal design internal temperature gradients exist between the laser die and the thermistor because of both ambient temperature changes and laser die heating. A change in ambient temperature consequently causes a change in the laser temperature and this laser temperature change results in a systematic error in the laser diode emission wavelength. Typically the laser emission wavelength changes by 5 pm for each centigrade change in ambient temperature. An ambient change of 30 C will result in a laser emission wavelength change of 150 pm which is the equivalent to approximately three line widths of the 1651 nm methane line. A change in ambient temperature by only one degree will typically result in 5 pm change in laser emission wavelength which is typically 10% of a gas absorption linewidth and unacceptable for TDLS spectroscopy.
Thermal changes in the TEC and laser current generating circuitry also cause TDLS systems to drift. This drift is relatively small but is important for applications requiring high precision and accuracy. Electronic components are sensitive to temperature and dissimilar metals in a circuit board create thermoelectric voltages that change with the temperature of the circuitry. Even in carefully designed circuitry ambient temperature changes result in changes in TEC control currents and laser currents that cause the laser emission frequency to drift when the ambient temperature changes.
Several methods have been proposed to stabilize the emission wavelength of diode lasers.    {T. Ikegami, S. Sudo, Y. Sakai, “Frequency Stabilization of semiconductor laser diodes” Artech House, (1995)}
The commonest method is to use a sample of the target gas contained in a reference cell.    {Van Well, B., Murray, S., Hodgkinson, J., Pride, R., Strzoda, R., Gibson, G. and Padgett, M., “An open-path, hand-held laser system for the detection of methane gas,” J. Opt. A—Pure Appl. Opt. 7, S420-S424 (2005)}
Gas reference cells are commonly used as absolute wavelength standards.    {Gilbert, S. L., Swann, W. C. and Dennis, T., “Wavelength standards for optical communications,” Proc. SPIE 4269, 184-191 (2001)}
When a gas reference cell is used to stabilize emission wavelength in a TDLS system, the system typically has an optical reference path that contains the gas reference cell. The system analyzer captures the spectrum of the sample gas and uses a feedback loop to stabilize the laser emission wavelength and prevent wavelength drift. This is known in the art as line centering. The system adjusts emission wavelength by changing either the laser temperature or the laser injection current. This method has the disadvantages of adding substantial opto-mechanical complexity and with even the most careful design can introduce optical interference effects and degrade system sensitivity. Adjustment of the laser current or temperature by the system during line centering also typically causes changes of laser light amplitude and instrument calibration.
Other laser wavelength stabilization methods use athermalised etalons and electrically stabilized optical resonators as wavelength standards. These methods share the same disadvantages as the gas cell wavelength standard.    {Ackerman, D. A., Paget, K. M., Schneemeyer, L. F., Ketelsen, L. J.-P., Warning, F. W., Sjolund, O., Graebner, J. E., Kanan, A., Raju, V. R., Eng, L. E., Schaeffer, E. D. and Van Emmerik, P., “Low-cost athermal wavelength locker integrated temperature-tuned single-frequency laser package,” J. Lightwave Technol. 22 (1), 166-171 (2004)}    {Sandford, S. P. and Antill, C. W., “Laser frequency control using an optical resonator locked to an electronic oscillator,” IEEE J. Quantum Elect. 33 (11), 1991-1996 (1997)}
A recent paper has disclosed a method of directly measuring the laser junction temperature by measuring the laser junction voltage. The junction voltage is used in a control loop to stabilize the laser temperature and emission wavelength.    {A. Asmari, J. Hodgkinson*, E. Chehura, S. E. Staines and R. P. Tatam, “Wavelength stabilisation of a DFB laser diode using measurement of junction voltage” Proc. of SPIE Vol. 9135, 91351A (2014)}    {A. Asmari, J. Hodgkinson*, E. Chehura, S. E. Staines and R. P. Tatam “A new technique to stabilise the emission wavelength of laser diodes for use in TDLS” FLAIR, 37, Florence (2014)}
This method stabilizes the laser emission wavelength but the stability is inadequate for sensitive TDL spectroscopy. The method also requires injection current modulation which could compromise the modulation levels required for sensitive WMS.
Optical interference fringes caused by reflection from optical elements degrade the sensitivity TDLS systems and causes thermal drift. A reflectivity of only 0.0025 will cause interference fringes of optical depth of 1% peak to peak.    {C. R. Webster, “Brewster-plate spoiler: a novel method for reducing the amplitude of interference fringes that limit tunable-laser absorption sensitivities” JOSA B, Vol. 2, Issue 9, pp. 1464-1470 (1985)}
In most TDLS systems reflection between the laser diode and its packaging and other optical components cause fringes within a TDLS system. Changes in the path length between optical elements and the laser, usually the result of thermomechanical changes, by as little as a fraction of a wavelength will cause the fringes and optical interference frequency to change. For example in a typical near infrared TDLS system only a one degree change in temperature changes the path between the first lens and the laser by approximately ten wavelengths. Optical interference changes caused by ambient temperature changes are consequently another important source of drift in TDLS systems. Line centering has no impact on the thermal drift caused by optical fringes in a TDLS system.